The Marin Board of Supervisors was right to reject the sole bid it received for building the much-anticipated first phase of the bike park at Stafford Lake Park. The initial piece of the 17-acre park has been estimated to cost $502,000, the first phase of a projected $850,000 project.

The county's problem is that when the first phase was put out to bid, only one proposal was received — for $1.4 million.

Either the county's estimate is overly optimistic or the concept of competitive bidding didn't work.

If the bid is closer to a true construction cost, the park could wind up costing more than $3 million. That's a lot more than was bargained for when the plan was approved by supervisors.

That's why supervisors decided to test the market and its estimate again.

The park is a promise county supervisors made to local mountain bike enthusiasts. It is supposed to provide bikers a place to race filled with jumps, berms and wood take-offs — a much better option than rogue trails that have been carved across Marin's open space lands.

Late last year, officials discovered a long mountain-bike trail built through woodlands in Tamalpais Valley, off the Miwok Trail.

In recent years, a 2.5-mile trail was found that had been created across the French Preserve in San Geronimo Valley. More than 60 trees were cut down to blaze the bike trail.

The environmental damage to public lands should not be tolerated. Voter passage of the 2013 county parks tax was vital for the county to provide more open space rangers to watch out for rogue trails.

But these were not the first trails discovered and supervisors, at the urging of local mountain bikers, agreed to a plan to provide a better alternative.

Park supporters have been successful in raising numerous donations for bike-oriented companies and local families.

While bike access to more single-track trails remains a well-debated issue at the Civic Center, supervisors hope that the bike park helps address the frustration of a growing bike community — including competitive youth teams — that many open-space paths remain off-limits to bikes.

It would certainly answer frequent complaints from competitive riders thaty they have to leave the county — the renowned birthplace of mountain biking — in order to ride.

The hope was that construction could start this summer.

Removing parking and making other cost reductions, including depending on volunteers to help build parts of the park, should help bring bids in closer to the county's estimates.

If the bids are not closer — a lot closer — to the county's estimate, supervisors need to take an even harder look at what it can deliver at the cost it has told the public.

Supervisors have sold the project as a park that will be mostly funded by private donations and sponsorships.

Perhaps taking another shot at getting competitive bids — and getting more than one — will help put construction of the park back on track.

We hope it works. Until this obstacle arose, the park has been a fine example of a private-public partnership.

But the park is not going to be built based on overly optimistic low estimates.

Supervisors have a promise to keep with the mountain bike community, but they also promised taxpayers that most of the park would be built with private donations.